The present invention relates to a high sensitivity gyroscope system with noise subtraction for reducing excess noise and random walk in the gyroscope system.
Fiber optic gyroscope systems typically use broadband sources such as superluminescent diodes (SLDs), or fiber superluminescent sources, to reduce Rayleigh scattering and polarization noise. These sources introduce an excess noise term, in addition to shot noise, due to their finite bandwidth, into the gyro output. This excess noise causes the performance of the fiber optic gyroscope systems to saturate, rather than improve, as the source power is increased. It is desirable to eliminate this excess noise component in the gyro output to achieve optimum gyroscope performance. The gyroscope system of the present application performs noise subtraction using a delayed reference signal from a source in order to reduce excess noise.
Broad-spectrum optical sources with stable spectra are required in fiber-optic gyroscopes to minimize coherent back-scattering noise and zero rotation drift due to the Kerr effect. Superluminescent diodes (SLDs) have been implemented in fiber gyroscopes, but they generally suffer from a high wavelength sensitivity to temperature, inefficient coupling to single-mode fibers, and a lack of immunity to optical feedback. Superluminescent sources exhibiting superluminescence or superfluorescence, have been observed in high-gain laser materials as a result of an essentially single-pass amplification of spontaneous photons without the use of an optical resonator. The search for practical SLSs has evolved toward wave guiding structures, which offer the advantage of high-energy confinement and therefore, large gains in compact, efficient devices.
Superluminescent fiber sources (SLSs) present several advantages over SLDs. First, the temperature stability of the SLS spectrum, in particular, its center wavelength, is far superior to that of the semiconductor devices, whose emission wavelength typically varies by about 0.05 nm/deg C. Second, the available power in an SLD is significantly less than the available power in a superluminescent fiber source (SLS). For example, in a typical SLD, the available power is approximately 30 mW, of which probably no more than a few milliwatts can be coupled to a single mode fiber. Third, in a practical system, unwanted spurious reflections from the source/system interface can greatly reduce the power which can be coupled to the system fiber. These reflections can be minimized in the SLS fiber device by splicing the source and system fibers with a fused glass-to-glass splice, which can not be realized with SLDs. Finally, the high conversion efficiency of the SLS fiber source and its broad character pump band make superluminescent fiber sources (SLSs) ideal in compact, laser-diode-pumped configurations.
Nd-doped single-mode fibers have been utilized as a superfluorescent source in SLSs to provide a wide bandwidth and high power for use in applications where sharp spectrum components from a broadband laser would be undesirable, such as in a fiber gyroscope. Further, systems have been designed in which an Nd-doped fiber is pumped by a laser diode to produce over 80 mW of superfluorescent output at a wavelength of 1060 nm. Further, high-power superfluorescent sources have been demonstrated which are pumped by a high power broad-striped diode laser. The output characteristic with pump power has permitted the modeling of the superfluorescent emission and a determination of the fiber constants characterizing saturation, spontaneous emission, and gain. Further, in these systems, evidence of lasing in the absence of external feedback has been observed in particular configurations and is shown to correlate with Rayleigh backscattering levels in the fiber.
A double clad, high brightness Nd fiber laser has been pumped by a GaAlAs diode array. As illustrated in FIG. 1, the fiber laser includes an Nd core in the center, a first cladding, which is approximately rectangular in shape, which allows for efficient pump light absorption and is made of glass, and a second cladding, which is made of a soft fluoro-polymer, which increases an acceptance angle for the pump light, and has a refractive index of 1.39. Further, the fiber laser includes an outer buffer coating which is a commercial hard polymer. Since the first cladding is mainly SiO.sub.2, the numerical aperture (NA) between the first and second cladding is approximately 0.4. The core contains 0.5 weight % of Nd.sub.2 O.sub.3, 3.8 weight % Al.sub.2 O.sub.3, and an NA of 0.16. Further, the dimensions of the first cladding are 110 microns by 45 microns, and the core is 4.8 microns in diameter. This configuration gives a ratio of the first cladding area to the core area of 274. The rectangular shape of the first cladding and its NA of 0.4 make is especially suited for high powered diode array pumps.
Excess noise is important with respect to broadband optical sources which are commonly used in fiber optic gyroscopes because the excess noise can limit the ultimate sensitivity of the device. Models for excess noise have been used to calculate the random walk coefficient due to shot and excess noise in a fiber gyro to demonstrate the impact excess noise in these sources will have on such a gyroscope. Experiments in this area have indicated the gyros utilizing SLD sources are not significantly impacted by excess noise due their limited output power (a few mW in a single mode fiber). However, a fiber source at 1.06 microns, for example, with its higher potential output power, is limited by excess noise.
Excess noise in a broadband source arises due to intensity fluctuations. Gaussian intensity fluctuations lead to a Bose-Einstein, rather than a Poisson, photoelectron distribution, which leads to an additional, or "excess" noise term in the mean square fluctuation of the photoelectron current. This phenomenon is represented by: EQU &lt;(.DELTA.I).sup.2 &gt;=2e&lt;I&gt;B+&lt;I&gt;.sup.2 B/.DELTA.v (1)
where &lt; &gt; represents a time average, I is the detector current, e is the electron charge, B is the electronic bandwidth, and Av is the optical linewidth of the source. In Equation (1), the first term is referred to as the shot noise term and the second term is the excess noise term.
Broadband sources have been used in optical gyros to reduce noise due to Rayleigh backscatter, Kerr effect, and polarization fluctuations. The minimal detectable rotation rate as limited by shot noise in a fiber gyro, is shown below. If both the shot and excess noise terms from Equation (1) are included, the result is: ##EQU1##
where .OMEGA..sub.min is the minimal detectable rate, R and L are the radius and length of the gyro coil, J.sub.0 =0.34 and J.sub.1 =0.58 are Bessel functions optimized for maximum sensitivity, c is the speed of light, and .lambda. and .DELTA..lambda. are the free space center wavelength and linewidth of the source. By dividing both sides of Equation (2) by .sqroot.B, a bandwidth independent constant characteristic of white noise, .OMEGA..sub.min /.sqroot.B, is obtained, usually referred to as the random walk coefficient and expressed as (deg/h)/.sqroot.Hz or deg/.sqroot.h. As illustrated in Equation (2), the random walk coefficient depends on the source linewidth, as well as, coil configuration, detector current, and source wavelength.
Excess noise measurements on several types of SLDs and a superfluorescent Nd-doped fiber source have been performed. The spectra of the Nd-doped fiber source, illustrated in FIG. 2 is unique in that the spectra has a long wavelength "tail" which broadens the linewidth considerably compared to the width of the main peak near 1.06 microns. The results of the comparison of excess noise between the several types of SLDs and the superfluorescent Nd-doped fiber source indicate that maximum SLD outputs available are in the transition region between shot noise and excess noise limited operation. However, since the Nd-doped source utilizes much higher powers (10 to 40 mW), the Nd-doped source falls in the excess noise limited regime, and as a result the signal-to-noise ratio (SNR) will not increase. This excess noise is important for fiber optical gyros because it imposes a limitation which does not arise in optical gyros which use narrow band sources.
In summary, fiber superluminescent sources (SLSs) are desirable due to their high power output and broad spectrum. However, because of their high output, "excess noise" prevents optimum performance. Therefore, a need in the art exists for an apparatus and method for solving the problem of excess noise, by utilizing a gyroscope system including noise reduction means for reducing the excess noise component in the gyroscope signal. The noise subtraction would, therefore, allow fiber superluminescent sources to be operated at even higher source powers.